purity: being in the clear before God, named as a condition a person is in, together with the counted days that condition covers
Every use of the word in this sense sits inside one law, Leviticus 12, about a woman after she has given birth. The law sets out two stretches of time. In the first she is out of bounds for worship, as she would be during her monthly period. Then a second and much longer stretch begins, and this word names it: she is counted clean again, and she waits out the remaining days before bringing her offering.
Two set phrases carry the word. The text speaks of "the blood of her purity", which is the surprising part, since the same kind of bleeding that made her unfit at the start is now described as belonging to her clean condition, so it no longer bars her. And it speaks of "the days of her purity", the counted span that runs until she comes to the entrance of the tent of meeting.
The difference from the neighboring words in this family is worth holding onto. The verb (טָהֵר) is the act of becoming clean or of a priest declaring someone clean. The adjective (טָהוֹר) describes a person, animal, or object as clean. The closely similar noun (טָהֳרָה) names the procedure a person goes through to get clean again, with washing, waiting, and an offering. This word names the condition itself, which is why days can be counted of it and blood can be described as belonging to it.